Writing fluid cartridges or writing fluid tanks are widely used to hold ink for tube writers. In use, as writing fluid is used up from the cartridge chamber, the fluid is replaced by air; for example, by air which enters from the ink equalization chamber present in tube writers.
While the writing fluid volume in the cartridge or tank changes only relatively little as the temperature changes, the same temperature changes cause essentially greater volume changes to any air present in the writing fluid holding chamber. Upon warming this air expands, causing a raised pressure, which pushes writing fluid into the body of the writer which must be taken up by the equalization chamber, in order to avoid a drip or blot. The equalization chamber must be so constructed as to be able to take up all writing fluid that is forced out, even in the unfortunate event that a great portion of the original writing fluid volume has already been replaced by air in the holding chamber of the cartridge or tank, without causing writing fluid to run out of the equalization chamber.
As soon as a writing fluid cartridge or tank is attached to a tube writer that has been thoroughly cleaned, a relatively large amount of writing fluid immediately runs out of the cartridge, or fill a corresponding volume within the body of the writer, so as to create a writing fluid connection between the cartridge and the tube writer, or the like. Consequently, a relatively large empty space is initially created in the cartridge, and immediately after the ink cartridge has been so attached. This space fills with air, by means of the equalication chamber, thus balancing the pressures. Even in the most ideal working conditions, temperature changes cause considerable changes in this air volume, which becomes even greater as more writing fluid is used from the cartridge.